The Northrop Electronic Rocket Tracking System (NERTS)

I started with the fundamentals: solid trigonometry for determining the altitude and position of a target based on sightings from two fixed and known locations downrange of the launch tower. John Porter, the advisor to the PRS student branch and my earliest mentor, helped me design the hardware to solve the equations in real time. This got rather complex and the system would eventually become the Northrop Electronic Rocket Tracking System, or NERTS. Papers I would present on the system would win me awards in student competitions of the Society of Automotive Engineers and the Institute of Aeronautical Sciences, but that’s later.

Another advantage of living in LA: you could find things like potentiometers that read out voltage as a function of the cosine of the angle the shaft was turned. I needed two of them: one for each station, and in fact everything we built had to be duplicated. But I also had to measure the cotangent of an angle and nobody made such a device, so I built them from wire-wound resistors. Each tracking mount moved in altitude – an arm went up and down – and azimuth – it rotated about the vertical axis– and the output of the potentiometers varied according to these motions. The circuitry was hooked up to a voltmeter that displayed voltage as a function of the potentiometer readings and, by taking into account the distance downrange of the stations (2,000 ft) and apart (also 2,000 ft) , the altitude could be determined.

The picture on the right shows me, dressed to the nines, with one of the tracking mounts and the plotting board. The camera is a Bolex 16mm with a 230mm (9x) telephoto lens, which I still have.

The cosine “pot” is the large round thing on the right and the voltmeter is inset on the upper right of the board. The smaller cylinder on the tracking mount is a “selsyn,” something else you could find in a surplus store in LA. They were used on aircraft to remotely aim gun mounts: moving the gunsight moved the attached master selsyns and the slaves on the gun mount moved it the same direction and amount. On NERTS, the masters were on the tracking mounts and the slaves were under the plotting board, where they rotated the long knitting needles on top. The intersection of the needles showed the location of the rocket over the firing range. The intent was to photograph the board with a camera similar to the one on the mount so that a record of position and altitude could be produced. We never got that far.

There is more to the NERTS story -- click on the plotting board. To return to the main page, click here.